1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like.
2. Related Background Art
In general, a copying machine has a function for reading an image on an original and the like and serves to form an image on a sheet on the basis or read image information. In recent years, there has been proposed a copying machine having a communication function for inputting image information sent from an external equipment.
Further, in general, a printer serves to form an image on a sheet on the basis of image information sent from an external equipment such as a computer, and a facsimile has generally a function for reading an image on an original and a communication function so that read image information sent to an external equipment and forms an image on a sheet on the basis of information sent from an external equipment.
In such image forming apparatuses, there is provided an image bearing member (photosensitive drum or the like), and a toner image formed on the image bearing member is electrostatically transferred onto a transfer material (such as a paper sheet) by means of a transfer charger, and, thereafter, an unfixed toner image on the transfer material is thermally fixed as a permanent image by means of a fixing device.
In the past, there have been proposed various fixing devices.
In general, fixing devices in which the toner image is thermally fused to be fixed to the transfer material, and, fixing devices in which the transfer material is passed through a nip between a pair of rollers at least one of which includes a heat source and the toner image is fixed to the transfer material by heat and pressure have widely been used.
In such fixing devices, in order to maintain good fixing ability, a maximum electric power available within a predetermined electric power range is applied to a fixing heater so that reduction of a surface temperature of the fixing roller is prevented and a time period possible to start copy (warm-up time period) for rising-up the device for preparation for a copying operation, for example, in the first use is reduced.
However, in the above-mentioned conventional case, there arose the following problems.
In an image forming apparatus in which potential control for effecting preparation of an image forming portion (for example, for stabilizing potential of a photosensitive member by driving a charger and a lamp) is performed during the warm-up of the fixing device, if the electric power used for the potential control is added, the total electric power will exceed the predetermined electric power range.
In order to suppress the total electric power below the predetermined electric power, during the warm-up of the fixing device, at the same time when the charger and the lamp are driven, the output of the fixing heater is reduced, and the temperature is increased up to a predetermined temperature possible to start copy while reducing the output of the heater even after the preparation of the image forming portion is finished and the charger and the lamp are turned OFF.
Accordingly, in dependence upon the reduction of the output of the fixing heater, the time period required for increasing the temperature up to the predetermined temperature possible to start copy is lengthened.
Particularly, in case of a high speed apparatus requiring a greater amount of heat, since the temperature possible to start copy is high and heat capacities of the fixing roller and the pressure roller are also great, a long time period is consumed for the preparing operation.